1. Field of the Invention
The present invention generally relates to a method and apparatus for indicating the presence of airborne contaminants and accumulation of airborne contaminants, such as organic and inorganic bases, and more particularly to predicting the life of filter systems for removing the airborne contaminants.
2. Prior Art
Gas adsorption beds are used in many industries to remove airborne contaminants, such as organic bases, to protect people, the environment and often, a critical manufacturing process for the products which are manufactured. A specific example of an application for gas adsorption beds is the semiconductor industry where products are manufactured in an ultra-clean environment, commonly known in the industry as a "clean room". The manufacturing process typically requires the use of substances such as solvents to be used in the clean room environment. The use of these substances presents a problem when vapors are formed during the process which may contaminate the air and other processes in the room if they are not properly removed. In addition, many environments have several gases that may naturally occur in the ambient air that may contaminate the products and/or processes and are not removed by normal particulate filters. Typical contaminants that are produced by such processes are airborne bases, such as ammonia, organic amines and N-Methyl-2-pyrrolidone.
To eliminate the problem, contaminated air is often drawn through a granular adsorption bed assembly having a frame and adsorption medium, such as activated carbon retained within the frame. The adsorption medium adsorbs the gaseous contaminants from the air flow and allows clean air to be returned to the clean room and/or process. It can be appreciated that the removal efficiency of such beds is critical in order to protect the processes and the products that are involved.
It can further be appreciated that since the removal process involves passage of air through an activated carbon bed that adsorbs or chemically reacts with the airborne contaminants, there is no measurable pressure change as occurs when particulate filters are loaded. Therefore, it is difficult to directly monitor the status and deterioration of the activated carbon bed. Monitors placed downstream may detect performance, efficiency, or when a failure has occurred and that the adsorption beds are spent. However, presently available sensors may not be sensitive enough to work at the contaminant threshold levels which are critical in the semiconductor industry and are often quite costly. A problem with sensors having acceptable sensitivity is that they are often specific to a single contaminant. Although such sensors may detect a low level of one contaminant, others may accumulate to high levels and remain undetected. However, once there is an indication that an adsorption bed is spent, it is often too late and the process or products have often been ruined or damaged.
Other systems have been devised which can monitor adsorption bed life such as placing the beds in series. When adsorption beds are placed in series, a sensor may be placed in series intermediate the two adsorption beds. Therefore, as one adsorption bed becomes spent and the sensor indicates the presence of a contaminant, the second adsorption bed is still effective and failures are prevented. However, such detection systems have several drawbacks. When two adsorption beds are used, the pressure drop is doubled. This may be critical in some applications. In addition, once the first bed has been indicated as being spent, the adsorption beds are normally rotated in a sometimes complicated manner. Such rotation increases the maintenance and down time of such a system. At other times, both adsorption beds may be changed out, thereby decreasing labor, but also shortening the useful life of the downstream adsorption beds as they are removed prior to being fully spent.
Other systems utilize a sensor placed directly in the adsorption bed. However, in very thin adsorption beds, such a sensor may take up valuable space. In addition, an interface for detecting the presence of contaminants within an adsorption bed requires seals and can be complicated and expensive.
It can be appreciated that if the filtered air can be distributed in a balanced, even manner over the adsorption beds, a reliable prediction of the expected useful duration of each bed would enable a longer change out interval period without failure. It can be appreciated that achieving the greatest possible change out interval without failure would decrease filter materials cost and labor costs utilized in changing the adsorption bed filters.
It is desirable to have an indication of the actual amount of contaminant that the filter beds have been exposed to based on a known filter capacity and being able to accurately predict an optimal change out period for the adsorption beds. Such a process is more precise if the actual flow passing through the filters is known and the prediction based on a flow which is proportional to the actual flow through the adsorption beds. By sampling upstream of the adsorption beds, an accurate prediction of the contaminants which are flowing to the adsorption beds can be made.
It can be seen then that an indicator system is needed that detects the cumulative levels of airborne contaminants. Such a system should be able to sample a proportional amount of airborne contaminants that are flowing past an adsorption bed device. Such an indication system should provide a clear visual indication of the bed usage and indicate when the adsorption beds should be changed. In addition, as contaminant concentrations may vary, the system should provide a real time indication of cumulative contamination levels for predicting the change out interval based on the actual contaminant flow past the adsorption bed. It can also be appreciated that such a system should provide for a variable safety factor to ensure that adsorption bed failures do not occur. Such a system should also be able to measure the presence and cumulative level of such contaminants in an environment and provide a visual indication. The present invention addresses these as well as other problems associated with indicating the presence of airborne contaminants.